RMEFFECTS OF DREDGINGOPERATIONS PROGRAM

US ArmyorTECHNICAL REPORT D-88-9

REFINEMENT OF COLUMN SETTLING TESTPROCEDURES FOR ESTIMATING THE

QUALITY OF EFFLUENT FROM CONFINED0DREDGED MATERIAL DISPOSAL AREAS

'byW) Michael R. Palermo, Edward L. Thackston

(V) Environmental Laboratory0

DEPARTMENT OF THE ARMYWaterways Experiment Station, Corps of EngineersPO Box 631, Vicksburg, Mississippi 39181-0631

DTICK ~r( ELECTE

.4hT ZQ9r

JDecember 1988if-- ~-~---Final Report

"--- .Approved For Public Release Distribution Unhioited

-w

89 1 31 020Prepared for DEPARTMENT OF THE ARMY

US Army Corps of EngineersWashington, DC 20314-1000

-t Under LEDO Work Unit 31775

OW

Destroy this report when no longer needed Do not return

it to the originator

Tre findings in this report are not to be construed as an official

Department of the Army position unless so designated

by other authorized documents

The contents of this report are not to be used for

advertising. publication or promotional purposes

Citation of trade names does not constitute an

official endorsement or approval of !he use of

such commercial products

The 0-series of reports includes publications of the

Environmental Effects of Dredging Programs

Dredging Operations Technical Support

Long-Term Effects of )redging Orepratons

Interagency Field Verification of Methodologies for

Evaluating Dredged Material Disposal Alternatives(Field Verificatain Program)

0W I i I I i

SECURITY CLASSIFICATIONtVTTHIS PAGE

F orm ApprovedREPORT DOCUMENTATION PAGE MB No 0704-0o1

Ia. REPORT SECURITY CLASSIFICATION lb RESTRICTIVE MARKINGS

Unclasaif LedZa. SECURITY CLASSIFICATION AUTHORITY 3. DISTRIBUTION IAVAILABILITY OF REPORT

Approved for public release; distribution2b. DECLASSIFICATION/DOWNGRADING SCHEDULE unlimited.

4. PERFORMING ORGANIZATION REPORT NUMBER(S) 5 MONITORING ORGANIZATION REPORT NUMBER(S)

Technical Report D-88,-9

60. NAME OF PERFORMING ORGANIZATION 6b. OFFICE SYMBOL 7a. NAME OF MONITORING ORGANIZATION

USAEWES (if applicable)

Environmental Laboratory6c. ADDRESS (City, State, and ZIP Code) 7b. ADDRESS (City. State, and ZIP Code)

PO Box 631

Vicksburg, MS 39181-0631

Ba. NAME OF FUNDINGISPONSORING Bb. OFFICE SYMBOL 9. PROCUREMENT INSTRUMENT IDENTIFICATION NUMBERORGANIZATION (if applicable)

US Army Corps of Engineers I8c. ADDRESS(City, State, and ZIPCode) 10. SOURCE OF FUNDING NUMBERS

Washington, DC 20314-1000 PROGRAMNO. IoPROJECT ITASK IWORK UNITELEMENT N. NO. NO. rCCESI N

11. TITLE (Include Security Classification) 1

Refinement of Column Settling Test Procedures for Estlm'ting the Quality of Effluent from

Confined Dredged Material Disposal Areas12. PERSONAL AUTHOR(S)

Palermo. Michael R.; Thackston, Edward L.13a. TYPE OF REPORT 13b. TIME COVERED 14. DATE OF REPORT (Year, Month, Day) 15. PAGE COUNT

Final re ort FROM , TO Ort 6 December 1988 4616. SUPPLEMENTARY NOTATION

Available from National Technical Information Service, 5285 Port Royal Road, Springfield,

VA 22161.17. COSATI CODES 18 SUBJECT TERMS (Continue on reverse if necessary and identify by block number)

FIELD GROUP SUB-GROUP Confined disposal ;' Effluent quality

-Contaminants; Suspended solids'.Dredged material",-,, Turbidity (cd > -,

19. ABSTRACT (Continue on reverse if necessary and identify by block number)

.. " Column settling test procedures are used in designing confined dredged material dis-

posal areas for effective sedimentation and initial storage of solids. This report

presents the results of a study in which the test procedures were refined to allow a pre-

diction of the effluent suspended solids concentration for conditions normally found when

saltwater sediments are encountered. For this case, the dredged material normally settles

by zone settling, forming a clarified supernatant in the confined disposal area. This

study determined that the settling behavior of the fine particles initially remaining in

the supernatant following zone settling is flocculent settling. Procedures were developed

for predicting effluent concentrations as a function of retention time and other pertinent

operating conditions. The ability to make such predictions is essential if total concen-

trations of contaminants in the effluent from confined disposal areas must be determined.

20. DISTRIBUTIONIAVAILABILITY OF ABSTRACT 21. ABSTRACT SECURITY CLASSIFICATION[ UNCLASSIFIEDIUNLIMITED 0 SAME AS RPT. 0 DTIC USERS Unclassified

22a. NAME OF RESPONSIBLE INDIVIDUAL 22b. TELEPHONE (Include Area Code) '22c. OFFICE SYMBOL

DD Form 1473. JUN 86 Previous edtions are obsolete. SECURITY CLASSIFJCATION OF THIS PAGE

Unclassified

Unclassifiled

SECURITY CLASSIFICAT!ON OF T7iS PAGE

PREFACE

This work was conducted as part of the Long-Term Effects of Dredging

Operations (LEDO) Program by the Environmental Laboratory (EL), US Army Engi-

neer Waterways Experiment Station (WES), Vicksburg, Miss. The LEDO Program is

sponsored by the Office, Chief of Engineers (OCE), US Army. This report was

written as part of work unit 31775, "Techniques for Predicting Effluent Qual-

ity of Diked Containment Areas."

The work was performed by Dr. Michael R. Palermo, Research Civil Engi-

neer, Environmental Engineering Division (EED), EL. Guidance and technical

review for this work were provided by Dr. Palermo's dissertation research com-

mittee: Drs. Edward L. Thackston, Frank L. Parker, Peter G. Hoadley,

Antonis D. Koussis, and Horace E. Williams, all of Vanderbilt University, and

Dr. Robert M. Engler, Program Manager, Environmental Effects of Dredging Pro-

grams, EL. This report was written by Dr. Palermo and Dr. Thackston, who par-

ticipated under an Intragovernmental Personnel Act (IPA) agreement. The work

was performed under the general supervision of Dr. Raymond L. Montgomery,

Chief, EED, and Dr. John Harrison, Chief, EL. Manager of LEDO within EL's

Environmental Effects of Dredging Programs was Dr. Engler. The Technical

Monitors for OCE were Dr. William L. Klesch, Dr. Robert W. Pierce, and

Mr. Charles W. Hummer. The technical reviewers were Mr. Daniel E. Averett,

Dr. Paul R. Schroeder, and Dr. F. Douglas Shields. This report was edited by

Ms. Lee T. Byrne of the WES Information Products Division, Information Tech-

nology Laboratory.

Commander and Director of WES was COL Dwayne G. Lee, EN. Technical

Director was Dr. Robert W. Whalin.

This report should be cited as follows:

Palermo, M. R., and Thackston, E. L. 1988. "Refinement of ColumnSettling Test Procedures for Estimating the Quality of Effluent fromConfined Dredged Material Disposal Areas," Technical Report D-88-9US Army Engineer Waterways Experiment Station, Vicksburg, Miss.

............ . .. . , mam m m mmu mNN m 1

CONTENTS

Pae

PREFACE ................................................................ 1

CONVERSION FACTORS, NON-SI TO SI (METRIC) UNITS OF MEASUREMENT .......... 3

PART I: INTRODUCTION .................................................. 4

Background....................................................... 4Purpose and Scope ................................................. 5

PART II: DREDGED MATERIAL SETTLING.................................... 6

Types of Settling ................................................. 6Dredged Material Settling Processes ............................... 7

PART III: REFINEMENT OF PROCEDURES FOR PREDICTING EFFLUENTSUSPENDED SOLIDS .............................................. 9

Modification of Column Settling Test Procedures ................... 9Settling Regime of Supernatant Suspended Solids ................... 10Refined Prediction of Effluent Suspended Solids Concentration ..... 14

PART IV: COMPARISON OF LABORATORY ESTIMATES AND MEASUREDFIELD DATA ................................................... 23

Field Data ........................................................ 23Column Tests ...................................................... 23Adjustment Factors for Turbulence and Resuspension ................ 25

PART V: RECOMMENDATIONS ................................................ 29

REFERENCES .............................................................. 30

APPENDIX A: RECOMMENDED COLUMN TEST PROCEDURES AND EXAMPLECALCULATIONS ............................................... Al

Recommended Column Test Procedures ................................ AlDetermination of Effluent Suspended Solids Concentration .......... A8Determination of Field Mean Retention Time ........................ A9Example Calculations .............................................. A10

2

*1

CONVERSION FACTORS, NON-SI TO SI (METRIC) UNITS OF MEASUREMENT

Non-SI units of measurement used in this report can be converted to SI

(metric) units as follows:

Multiply By To Obtain

acres 4,046.873 square metres

acre-feet 1,233.489 cubic metres

cubic feet 0.02831685 cubic metres

feet 0.3048 metres

gallons (US liquid) 3.785412 cubic decimetres

inches 2.54 centimetres

Accession For

NTIS GRA&IDTIC TAB

U:' nnounced14 Jus Cifation0

Distribhution/Avnil,-bility Codes

Avaril and/orDist pecil

3

[ .

N ..

REFINEMENT OF COLUMN SETTLING TEST PROCEDURES FOR ESTIMATING

THE QUALITY OF EFFLUENT FROM CONFINED DREDGED MATERIAL

DISPOSAL AREAS

PART I: INTRODUCTION

Background

1. The quality of effluent from confined disposal areas is strongly

dependent on the concentration of total suspended solids in the effluent.

Since most of the contaminants of interest are associated with particles, the

concentration of suspended solids in the effluent will be a major factor

influencing the total concentration of contaminants on a weight per unit vol-

ume basis. A modified elutriate test has been developed (Palermo 1984, 1986)

that determines the concentration of dissolved contaminants and the contami-

nant fraction of the suspended solids likely to be present in the effluent.

An accurate estimate of the concentration of suspended solids in the effluent

must be made so that it can be used in conjunction with results of the mod-

ified elutriate test in order to predict the total concentration of contami-

nants in the effluent.

2. Montgomery (1978) developed procedures for estimating the total sus-

pended solids concentration in confined disposal area effluents when the

dredged material slurry exhibited a flocculent settling behavior. However, no

procedures were proposed to allow prediction of the solids concentration in

the effluent if the dredged material slurry exhibited a zone settling behav-

ior. The settling behavior of the low concentrations of suspended solids (on

the order of 100 mg/1) in the semi-clarified water above the interface for

this settling case was not known at that time.

3. When slurries undergo zone settling in a laboratory test column,

elmost all of the solids are entrapped in a loose open matrix that settles as

a single mass. However, a few colloidal solids that are not trapped in the

matrix remain in the semi-clarified supernatant above the interface. In

addition, as the mass settles, it displaces water from below, which must move

upward through the voids in the settling mass. This upward water velocity

shears some loosely bound colloids from the settling mass and carries them

4

into the supernatant, causing the suspended solids concentration to rise in

the initial stages. Higher slurry concentrations produce smaller void spaces

and higher resulting upward water velocities, causing more solids to be car-

ried into the semi-clarified supernatant. In addition, under field condi-

tions, wind and advective flow generate turbulence that inhibits sedimentation

and resuspends some solids, increasing the effluent suspended solids concen-

tration above that predicted by a quiescent laboratory column settling test.

4. Montgomery (1978) did not propose any method of quantifying the sus-

pended solids concentration in the supernatant, which in a confined disposal

area becomes the effluent concentration. He stated only that the concentra-

tion should be below 1 to 2 g/t, low enough to satisfy most of the effluent

discharge permit standards common at that time. However, today, with many

discharge permits limiting suspended solids concentrations in the effluent to

below 0.1 g/t, this approach is not sufficient. Therefore, Palermo (1984,

1986) devised a method to predict the suspended solids concentration in the

supernatant 3s a function of retention time, based on the results of the lab-

oratory flocculent settling test.

Purpose and Scope

5. The objective of this study was to develop a technique that could be

used to predict the suspended solids concentration in the effluent from a con-

fined disposal area in which the bulk of the slurry mass settled by zone set-

tling. The work involved a modification of the column test procedures

dei, 1oped by Montgomery (1978) and the develepmert and verification of appro-

priate methods of data analysis.

6. Procedures were designed to be as simple as possible, while still

remaining soundly grounded in fundamental principles and being able to produce

reliable, accurate results. They were also designed to simulate Rct"nl f pld

conditions as closely as possible.

7. This technical report includes a description of the experimental

work, a description of the experiments conducted to verify the applicability

of the predictions to actual field conditions, an outline of the recommended

procedures, and an example illustrating the use of the procedures. The pro-

cedures can now be used by US Army Corps of Engineers field offices and permit

applicants in the evaluation of confined dredged material disposal activities.

5

PART 11: DREDGED MATERIAL SETTLING

Types of Settling

8. This background material will be familiar to the civil or chemical

engineer who has advanced training in physical/chemical water treatment pro-

cess design, but not to most general civil engineers. Therefore, it is pro-

vided as an aid to understanding the descriptions of the tests and the

rationale behind the decisions and choices that were made in the development

of the procedure.

9. Four different types of settling are generally recognized. The

type that occurs in any given suspension is a function of both the type of

particle involved, particularly its surface characteristics, and the concen-

tration of particles at a given time. The four types are listed below:

a. Discrete settling (Type I). The particle does not interactduring settling. Each particle maintains its individuality anddoes not change in size, shape, or density while settling.Each particle settles as if it were alone and isolated.

b. Flocculent settling (Type I). The particles flocculate andagglomerate during settling. As the particles grow in size,they decrease in density because of entrained water, but theyusually settle faster.

c. Zone settling (Type III). The concentration of particles is sogreat that they touch adjacent particles in all directions andmaintain their spatial relationship, settling as a mass or openmatrix. They usually exhibit a definite interface between thesettling particles and the clarified liquid above. The par-ticle matrix settles more slowly than the individual particlesof the same size and density because the quantity of waterbeing displaced by the settling particles is so great that theresulting upward velocities of the displaced water reduce theeffective downward velocity of the particle mass.

d. Compression settling (Type IV). The concentration is so greatthat the particles rest on each other and mechanically supporteach other. The weight of the particles above slowly com-presses the lower layers, increasing the pore pressure andsqueezing out the water. This is also sometimes called thick-ening or compaction. In water treatment plants, the settlingis sometimes aided by slow stirring to break up the bridgingaction of the particles, but this is impossible in confineddisposal areas, which must rely on gravity alone.

10. The relation of the different types of settling to type of particle

and concentration of particles is shown in Figure 1.

6

L:U

LOW I - DISCRETE II-FLOCCU-SETTLING LENT

SETTLING

0

Z

U)

-J0

~IV - COMPRESSION

HIGH

GRANULAR FLOCCULENT

PARTICLE CHARACTERISTICS

Figure 1. Types of settling

Dredged Material Settling Processes

11. From Figure 1 it can be seen that discrete settling occurs only in

suspensions with low concentrations of granular particles. This occurs in a

confined disposal area only with the small fraction of larger particles (sand

and gravel) occasionally encountered and with intrusions such as bricks,

crockery, shells, broken tools, and household items that were thrown or washed

into the waterway. It never occurs with hydraulically placed fine-grained

dredged material, because the concentrations of the influent are so high (50

to 200 g/1) and because most of the particles (clay, silt, organic matter) are

naturally flocculent. All of the other three settling processes may occur

simultaneously in a confined disposal area, and any one may control the design

of the confined disposal area.

12. Dredged material slurries will initially exhibit either flocculent

settling or zone settling, depending primarily on the slurry concentration,

particle type, and salinity of the water. Saline slurries with salinity

greater than 3 ppt will usually exhibit zone settling, because the dibG-aved

ions act as a coagulant. These ions compress the electrical double layer,

reduce the effective distance over which the natural repulsive surface forces

are effective, and allow sediment particles to touch enough adjacent particles

that a loose open matrix is formed, which settles as a mass. Freshwater

7

slurries usually exhibit flocculent settling, but may exhibit zone settling if

concentrations are high enough or if the particle surface characteristics are

flocculent enough.

13. Regardless of whether the upper layers of the containment area ini-

tially exhibit flocculent or zone settling, the lowest layers will exhibit

compression settling, or thickening. When the slurry approaches the bottom

and the concentration rises, successive layers will begin to rest on and be

supported by the bottom and then each other, much like an accordion being

slowly lowered onto a hard surface. The change from flocculent or zone set-

tling to compression settling, at which the bottom begins to provide some

physical support, occurs at a concentration of approximately 200 to 300 g/k

for most dredged material slurries.

8

PART III: REFINEMENT OF PROCEDURES FOR PREDICTING

EFFLUENT SUSPENDED SOLIDS

Modification of Column Settling Test Procedure

14. McLaughlin (1959) developed procedures to determine the settling

regime (discrete, flocculent, or zone) of slurries. A series of laboratory

settling tests using McLaughlin's procedures was conducted on representative

fine-grained sediments to investigate the settling regime of the supernatant.

15. The 8-in.* settling column previously recommended for confined

disposal area design (Montgomery 1978; Palermo, Montgomery, and Poindexter

1978) was used to conduct settling tests on sediments from Mobile Harbor,

Alabama (composite); Black Rock Harbor, Connecticut; and Yellow Creek,

Mississippi. Generally, the standard testing procedure described by

Montgomery for freshwater sediment was used. However, when zone settling of

the slurry was observed, with a clearly defined interface, samples were taken

from side ports above the interface. This procedure had not previously been

performed.

16. The tests were conducted by mixing the sediment and water from the

dredging site to concentrations within the range expected for disposal area

influents. The slurry was then placed in the columns and allowed to settle.

For those sediments in which an interface formed, a sample of the supernatant

was taken from the uppermost port as soon as the settling interface fell below

the port far enough to allow sample withdrawal without disturbing the inter-

face. For all tests, this occurred within a few hours of the initiation of

the test. Samples were then taken from all ports above the falling interface

at time intervals of approximately 6, 12, 24, 48, and 96 hr, continuing to

15 days or until the supernatant suspended solids concentration indicated

essentially no further removal of suspended solids through sedimentation. The

port samples were analyzed for concentration of total suspended solids in the

supernatant.

17. The Yellow Creek sediment was from a fresl.water environment, and

the tests were run at initial slurry concentrations of 33 and 148 g/k. Mobile

* A table of factors for converting non-SI units of measurement to SI

(metric) units is presented on page 3.

9

Harbor and Black Rock Harbor sediments were from saltwater environments, and a

range of initial slurry concentrations was tested. Additional tests were also

later run using Mobile Harbor sediment to check the reproducibility of

results.

Settling Regime of Supernatant Suspended Solids

18. The samples from the upper sampling ports made possible the deter-

mination of settling behavior in the supernatant using the graphical procedure

of McLaughlin (1959). McLaughlin describes the use of concentration profile

diagrams for analysis of quiescent settling test data. The fraction of sus-

pended solids remaining in a suspension, 0(z,t) , is plotted versus the depthbelow the fluid surface, z , for various sampling times, t , as shown in

Figure 2, and smooth curves are drawn through the data points, as illustrated

by the solid lines on Figures 2 through 4.

19. One will note that, in Figures 2 through 4, all of the 0 versus

z lines go through the origin. In other words, there is a zero solids

concentration at zero depth at any time greater than zero. This boundary

condition, originally stated by McLaughlin (1959), was justified on theFRACTION OF SUSPENDED SOLIDS REMAINING O(zt)

04

CONSTANT z/

01 AT T,

II

/L TiI- /

S / 3

~ 1 24 DEPTH=D

T3 T2

Figure 2. Typical concentration profilediagram

10

PERCENT OF INITIAL CONCENTRATION, 0 (z, )

0 10 20 30 40 50 60 70 80 q0 100

0 1 1 1 ! I '

z/t = 0.01 FT/HR

0.2

z/t O02 FT/HR

0.4 z/t = 0.03 FT/HR

0.6 - / f14 HR

0.8

x Co = 148 g/ Q

10 -

LEGEND/ 24 HR O 1'HR

1:R 24 HR48 HP

1.4 -H 48 HR 96 HR

1+ ,/ 096 HR 1 168 HR

1.6

T= 168 HR

1.8

Figure 3. Concentration profile diagram for particles above

the interface for Yellow Creek sediment

I I -

PERCENT OF INITIAL CONCENTRATION, (z, t)

0 10 20 30 40 50 60 70 80 90 100

0

1.0I--

. 5

1.5 -T = 7HRLEGEND04HR

o 7 HR

2.0 -10 HR

T =

46 HR *22 HR

V 17 28 HRT =28 HR 0 46 HR

CO~ ~ T 28 10H/R T=22H

2.5

Figure 4. Concentration profile diagram for particles above

the interface for Mobile Harbor sediments (composite)

grounds that, after any finite time, gravity will cause even the smallest

particles to settle some small distance away from the surface. However,

Brownian motion effects, air currents, thermal currents, currents caused by

hindered settling, and other similar effects will cause some suspended solids

to remain in suspension at or near the surface for long periods of time in a

real situation. This causes some difficulties in fitting smooth curves of

continuous mathematical functions through all points and in extrapolating them

to the surface and to the interface.

20. A digital computer program fit a continuous function to the data

points and computed the areas by numerical integration to determine the

re.oval percentages. The available equations are not always able to closely

fit all of the data points and also have the concentration at the surface

decrease continuously as a function of time. The curves do not really go

through the origin, because the low concentrations of colloidal solids

12

remaining in the supernatant after several hours of settling are not con-

trolled primarily by gravity forces and are as likely to be very near the sur-

face as anywhere else.

21. The experience of this project generally showed that:

a. The lines were usually concave downward, even if only veryslightly.

b. The lower portion of the line was sometimes almost straight andalmost vertical, but always sloped slightly away from the leftmargin at higher values of z .

c. The curves rarely if ever reversed direction.

d. Curvature was usually much greater near the surface than atgreater depths.

22. A family of lines of equal z-to-t ratio can also be constructed.

Such curves represent the concentration as seen by an observer by starting at

the surface of the suspension at initial t - 0 and descending at constant

velocity. (These are the dashed lines on Figures 2 through 4.) McLaughlin

(1959) observed that when neither hindered nor flocculent settling is occur-

ring, a line of constant z/t is straight and parallel to the z-axis. If

flocculent settling occurs, the z/t line slopes toward the z-axls (converges

at higher z values) as shown in Figure 2. If hindered settling occurs, the

line slopes away from the z-axis (diverges at higher z values).

23. The Yellow Creek sediment was from a freshwater environment, and

the entire slurry initially exhibited flocculent settling behavior. An inter-

face was then observed to form about 10 hr after the initiation of the 148-g/i

test. As expected, the analysis of samples taken above the interface also

showed that these solids exhibited flocculent behavior. The concentration

profile diagram for samples taken above the interface is shown in Figure 3.

Results for the 33-g/i test were similar. Note that Figure 3 was constructed

with 0 - 100 percent corresponding to the initial sample taken from the

uppermost port. The rationale for this approach is discussed in detail in

later paragraphs.

24. The Mobile Harbor and Black Rock Harbor sediments are from salt-

water environments. The slurries exhibited zone settling behavior at the

concentrations tested, with clearly defined interfaces forming between the

supernatant and the more concentrated slurry. However, the concentration pro-

file diagram indicates that the solids remaining in the supernatant exhibited

flocculent settling for all initial concentrations tested. A representative

13

... ....... . . .. .-- -.r - ,Jil I I l~ il ~ i!!!or

concentration profile diagram for the Mobile Harbor tests is shown in Fig-

ure 4. All z/t curves converge with the z-axis at higher z values.

Results for other concentrations in the Mobile Harbor and Black Rock Harbor

tests were similar. These data indicated that flocculent settling occurs at

concentrations below 100 mg/t for fine-grained dredged material.

Refined Prediction of Effluent SuspendedSolids Concentration

Data analysis

25. The flocculent settling behavior exhibited in supernatant water

greatly simplified the development of an appropriate method for data analysis

and prediction of effluent suspended solids concentrations. McLaughlin (1959)

utilized a graphical approach for flocculent settling data analysis using the

concentration profile diagram to compute the removal ratios for any given

retention time. A modification of this approach was developed to allow com-

putation of the concentration of suspended solids in the supernatant of the

column tests, and consequently, the prediction of suspended solids concentra-

tions in the effluent from the confined disposal area.

26. The procedure involves the determination of areas to the left and

right of the concentration profile line for a given retention time. The areas

are determined using the zero concentration and the initial concentration of

the test as the left and right vertical boundaries, and the zero depth and a

specified depth of averaging as the top and bottom horizontal boundaries. The

area to the right of the curve (area 0, 3, 4, 0 in Figure 2) divided by the

total area (area 0, 1, 2, 4, 0 in Figure 2) when multiplied by 100 will equal

the suspended solids removal percentage, Rt , at time T through the depth,

D . The percentage of initial solids remaining at time T, P is simplypw t

100-Rt . Pt is a depth-integrated value over the depth of averaging under

consideration.

27. This approach can be directly applied for the case of flocculent

settling of the entire slurry mass, as recommended by Montgomery (1978). How-

ever, for the case of zone settling of the slurry mass, the McLaughlin (1959)

approach must be applied only to the flocculent settling regime in the clar-

ified water above the interface. An initial sample can be taken only when the

settling interface clears the uppermost port. If the suspended solids

14

0'

concentration in this sample is assumed to be 0 = 100 percent , the removal

percentage, R 1 , and the percentage remaining, P1 , at TI may be calcu-

lated. The computation of subsequent removal ratios and percentages at other

retention times can be accomplished in a similar manner. These data may be

used to develop a relationship between removal percentage versus time as

described by Montgomery (1978), or as suspended solids concentrations remain-

ing versus time. The sensitivity of the results to the assumed initial con-

centration of suspended solids, initial slurry concentration, and depth of

averaging are discussed below.

Effect of initial supernatant

suspended solids concentration

28. To develop a concentration profile diagram, the removal percentages

must be expressed in terms of some initial concentration. McLaughlin (1959)

used the true initial concentration, but for the zone settling supernatant,

this is not measurable, so some initial concentration of supernatant suspended

solids must be assumed. The effect of the assumed initial suspended solids

concentration can be shown using the results trom the Mobile Harbor test run

at an initial slurry concentration of 108 g/k. An initial sample of the

supernatant was taken when the falling interface cleared the uppermost port at

T - 4 hr. The concentration of this sample was 110 mg/i. The concentration

profile diagram for this test is shown in Figure 4 with -= 100 percent cor-

responding to 110 mg/i. It is obvious that the measured value at 4 hr is not

a true value of initial suspended solids concentration in the supernatant.

However, using the first measurement as the reference value is convenient for

purposes of data analysis. Also, any higher value representing an earlier

time that could be calculated by some extrapolation method would have little

theoretical or practical advantage, since a few hours would be necessary for

sufficient supernatant volume to develop for flocculent settling processes to

begin.

29. The sensitivity of this assumption was shown by comparing rela-

tionships of total suspended solids remaining versus time calculated using

various values for the initial concentration. Table 1 summarizes the data for

an initial suspended solids concentration of 110 mg/i (the first real measure-

ment) and assumed values of 150 and 200 mg/i. The graphical determinations of

removal percentages were made using the general procedure as described previ-

ously, with the assumed value for initial concentration used to construct

15

f.or

Table I

Supernatant Suspended Solids Remaining for Various Assumed

Initial Supernatant Suspended Solids Concentrations

Mobile Harbor Sediment, Initial Slurry Concen-

tration 108 g/I, Depth of Averaging I ft

Assumed Initial Concentration in Supernatant10 mg/i 150 mg/1 200 mg/i

Time Removal Remaining' Removal Remaining Removal Remaininghr Percentage SS* (mg/1) Percentage SS (mg/i) Percentage SS (mg/i)

4 15 94 34 99 51 98

7 46 59 62 57 71 58

10 60 44 72 42 79 42

22 76 26 83 25 87 26

28 87 14 90 15 93 14

46 91 10 94 9 95 10

* SS - suspended solids.

concentration profile diagrams with 0 = 100 percent corresponding to the

assumed initial concentration. The graphical determinations were made for

D = 1.0 ft. The total suspended solids values were then calculated for eachpw

time by multiplying the percentages remaining by the assumed initial con-

centration. Exponential curves were fit to the data and are shown in Fig-

ure 5. These curves indicate essentially no effect due to the different

assumed values for initial suspended solids in the supernatant. This occurred

because the assumed initial concentrations greater than the first measured

value of 110 mg/1 placed the 0 = 100 percent vertical boundary for the

graphical procedure well to the right of the concentration profiles. The

areas bounded by the removal curves and thus the computed concentrations

remaining were therefore not substantially changed. It is therefore recom-

mended that the measured suspended solids concentration in the first port sam-

ple be used as the initial concentration for purposer of future data analysis.

Effect of depth of aver-aging on concentrations remaining

30. Under quiescent settling conditions, the percentage of suspended

solids remaining in the supernatant water increases with the depth of

16

I.

120

100

so LEGEND

,-.-,, INITIAL SUPERNATANT SOLIDS = 110 MG/L- e----.. INITIAL SUPERNATANT SOL!DS = 150 MG/L

60 INITIAL SUPERNATANT SOLIDS = 200 MG/L0(J

0

O

0

10 2 3 40 50

RETENTION TIME IN HOURS

Figure 5. Relationship of predicted effluent suspended solidsconcentration versus time for various assumed initial super-

natant concentrations, Mobile Harbor column test

averaging and decreases with time. This is clearly indicated by the shape of

the concentration profile diagrams. The sensitivity of the predicted remain-

ing suspended solids concentration to this variable was shown by comparing

relationships of total suspended solids versus time for several depths of

averaging, using the results from the same Mobile Harbor test with an initial

slurry concentration of 108 g/1. Table 2 summarizes the data for depths of

averaging of 1, 2, and 3 ft. The data were calculated using the first port

measurement as the initial concentration, as recommended above. Curves were

fit to the data as described above, and they are shown in Figure 6. The com-

parison shows that, for a given retention time, the suspended solids remaining

increase as the depth of averaging increases. For example, at a retention

time of 20 hr, the predicted concentration increases from approximately 25 to

40 mg/t as the depth of averaging increases from I to 3 ft. This magnitude of

difference shows that the depth of averaging must be carefully considered in

the prediction of effluent concentrations using column test results.

31. The appropriate depth of averaging for use in prediction should be

the best estimate of the depth of influence of the discharge weir of the

17

Table 2

Supernatant Suspended Solids Remaining for Various Depths of

Averaging, Mobile Harbor Sediment, Initial Slurry

Concentration 108 gI/, Initial Supernatant

Suspended Solids Concentration 110 mg/.

Depth of Averaging1.0 ft 2.0 ft 3.0 ft

Time Removal Remaining Removal Remaining Removal Remaining

hr Percentage SS* (mg/t) Percentage SS (mg/) Percentage SS (wg/t)

4 15 94 8 101 5 105

7 46 59 36 70 31 76

10 60 44 49 56 42 64

22 76 26 69 34 61 43

28 87 14 79 23 75 28

46 91 10 87 14 84 18

* SS suspended solids.

125

100 LEGEND

0- ....-- 0 DEPTH OF AVERAGING = 1.0 FTz- DEPTH OF AVERAGING = 2.0 FT

75 -m , DEPTH OF AVERAGING = 3.0 FT

o%

0

3CL

W 25

0 10

RETENTION TIME IN HOURS

Figure 6. Relationship of predicted suspended solidsconcentration versus time for various depths of

averaging, Mobile Harbor column test

18

i-r

ll "">A

confined disposal area under field conditions. The effluent discharged from a

confined disposal area will contain suspended solids that are drawn from the

ponded water in the disposal area. The depth of influence from which water is

withdrawn is governed by the characteristics of the weir structure and the

flow rate and ponded conditions within the disposal site, especially near the

weir.

32. Walski and Schroeder (1978) showed that the depth of the withdrawal

zone or depth of influence is highly dependent on the density gradient of the

fluid in the pond. For a dredged material undergoing zone settling, there is

a clearly defined interface between settled material and supernatant water,

essentially forming a two-zoned system. Field data collected by Walski and

Schroeder (1978) indicated that the depth of influence for this case is essen-

tially equal to the ponded depth, or depth to the interface. It is therefore

recommended that the assumed depth of averaging used in the prediction of

effluent suspended solids be selected as that equal to the anticipated depth

of ponding at the weir structure. An exception could occur in the case of a

confined disposal area with great depth, low discharge rate, and long dis-

charge weir.

Effect of initial slurry concentration

33. Montgomery (1978) showed that zone settling velocity was a function

of the concentration of the slurry tested. The effect of initial slurry con-

centration on concentrations of suspended solids remaining in the supernatant

was investigated by comparing results of the column settling tests conducted

with Mobile Harbor sediment. Initial slurry concentrations of 58, 1(B and

155 g/f were tested. The test data were analyzed using the initial port sam-

ple as the initial supernatant concentration, as discussed above, and a depth

of averaging of I ft. The reduced data for all three tests are summarized in

Table 3. Curves were fit to the data sets and are shown in Figure 7. The

results show that the suspended solids remaining increase as the test slurry

concentration increases, as shown in Figure 7. Greater slurry concentrations

produce smaller void spaces and greater upflow water velocities in the pores,

expelling more solids. For example, at a retention time of 24 hr, the pre-

dicted supernatant suspended solids concentration is 26 mg/i for an initial

slurry concentration of 155 g/, but only 13 mg/i for 58 g/L. This magnitude

of difference shows that the initial slurry concentration for the test should

be carefully selected. It is recommended that the initial slurry

19

.. . . .. • • • lil I I I ) I |

Table 3

Supernatant Suspended Solids Remaining for Column Tests Conducted

at Three Slurry Concentrations, Mobile Harbor Sediment

SlurryConcentration Time Removal Remaining

g/L hr Percentage SS* (mg/ij

58 1.25 6 1183.5 13 1097 49 64

14 77 2924 90 1348 94 8

108 4 15 947 46 59

10 60 4422 76 2628 87 1446 91 10

155 3.5 5 1716 60 729 68 5814 76 4327 85 2748 91 16

SS = suspended solids.

concentration for the test be as close as possible to the anticipated field

influent concentration. This recommendation is also consistent with the

selection of initial slurry concentration for conducting modified elutriate

tests.

Test replicability

34. The reproducibility of the column test data produced by the test

procedures recommended above was determined by conducting a set of replicate

column settling tests using sediment from Mobile Harbor. Three columns were

simultaneously filled with slurry using a manifold device to assure that

essentially equal concentrations were placed in the columns. The initial

slurry concentrations were 56 g/. Samples were taken from all three columns

at 2, 4, 8, and 24 hr. The resulting concentration profiles are shown in

Figure 8. The plotted points are labeled A, B, or C indicating the test

replicates. The plotted data show that there was little difference in the

replicate tests. Small differences always occur because of experimental error

20

9

200

15s0

z~LEGEND

0 100 4,..... INITIAL SLURRY CONCENTRATION = 58 G/L6---- INITIAL SLURRY CONCENTRATION = 108 G/L

w U-U INITIAL SLURRY CONCENTRATION = 155 GILa %

0

0 10 20 30 40 50 60RETENTION TIME IN HOURS

Figure 7. Relationship of predicted suspended solids concentrationversus retention time for various initial slurry concentrations,

Mobile Harbor column tests

in sampling and gravimetrically determining small concentrations of suspended

solids. Furthermore, since some degree of judgment is normally required when

drawing concentration profiles, the curves resulting from the three tests are

practically identical. It is therefore apparently not necessary to perform

more than one column settling test solely for purposes of predicting the

effluent suspended solids concentration.

21

SUPERNATANT CONCENTRATION d, (Zit). mg/I

0 20 40 00 so 100 120 140 160

AB C rBC BC 0 0 0B ft H A

C 0 0A AU a

AA A

AB 0C ~ ~ Bc

AAU A C BU0 0 0

CB

&A arn A c B* 0 0

AB B C

AA A

T~ ~ 24 R HRa A

B C2

LEGENDT~2 - 3 - T -2 HRT, 0 - T 4 HR

-a * -T- 8 H-R

T2 -T 24 HR

Figure 8. Concentration profile diagram for replicatecolumn tests, Mobile Harbor

22

PART IV: COMPARISON OF LABORATORY ESTIMATES

AND MEASURED FIELD DATA

Field Data

35. Comparisons of effluent suspended solids concentrations predicted

using the modified column test and the data analysis procedure described above

with mean measured field values were made for confined disposal areas at five

sites. These sites were Mobile Harbor, Alabama; Savannah Harbor, Georgia;

Norfolk Harbor, Virginia; Black Rock Harbor, Connecticut; and Kings Bay,

Georgia. The mean field effluent suspended solids concentrations, mean reten-

tion times as determined by dye tracer tests, and mean ponding depths for the

five disposal areas are tabulated in Table 4. The variables were measured by

the best methods available at the time and appropriate for field conditions,

but are recognized as somewhat imprecise. A full description of the field

data collection was given by Palermo and Thackston (1988).

Column Tests

36. Column settling tests were conducted on samples from each site for

the purpose of developing a relationship between predicted effluent suspended

solids and retention time. Tests were conducted on grab samples oi sediments

and water from the sites where the dredges were operating at the time that the

field sampling was begun. The sediment samples were mixed with site water to

concentrations approximating the known field mean influent concentrations.

The full details of the sampling program were described by Palermo (1986).

The initial slurry concentrations for each test are tabulated in Table 4.

Samples were extracted at side ports as soon as the interface cleared the

ports. Data were analyzed as previously described using the modified

McLaughlin analysis. The suspended solids concentrations remaining above the

depths corresponding to the known mean ponded depths In the immediate vicinity

of the weirs for the respective sites were determined for each time of sam-

pling. Exponential curves of suspended solids as a function of time were fit

to the data, and the predicted values for effluent suspended solids corre-

sponding to the known mean retention times of the five disposal areas, as

23

ll i l i l l l

4j .

-1 v 4 -.40040 0 )" :0

0

5~04 4 50 0

I CA5.

00441 ~~45.4.u4 . e4(

'mC . ~ s es0C. '45w

Cfl~a r4 54cc I

1. 0

~~4. 0 . 1

45 4

40 n0 4.0

0~~5.. '*.*t C 'A 0 4 -440u 0 '4 's 4. r045

"0 0 an to 40 4 0 ImI

41 0

*c 04054 0 C0 0 CA

V J.-2. Uss

r54 4.0.*.'V o 0 41.

4 1 .

0 4S45 W004 .0.40 4 o

0 0Z0 0 .4

'4u 1- 0 '

'5 r. -1 0 0

0045. to 0

0.~~0 "0O.5£ 4

;f4.0 04 404

00

24

determined by dye tracer tests, were determined. Results for each area are

tabulated in Table 4.

Adjustment Factors for Turbulence and Resuspension

37. The refined approach for prediction of effluent suspended solids

described above assumes that the confined disposal area is well designed and

operated, the weir has sufficient crest length, and ponding conditions are

such that resuspension of settled material is avoided. Good design assures

adequate ponded surface area and sufficient storage for the zone settling pro-

cess to concentrate the dredged material, if the entire slurry mass undergoes

zone settling. However, the mean field effluent concentration for well-

designed and well-operated sites would likely be higher than that indicated by

quiescent laboratory tests. Quiescent conditions are optimum for efficient

settling, but are impossible to achieve in the field. Flow of water through

the area produces advective velocities, which generate turbulence. In addi-

tion, wind produces surface shear, which induces even higher velocities.

These effects decrease settling efficiency and increase field effluent sus-

pended solids over lab column suspended solids for identical settling t1mes.,

In addition, some solids that settle when winds are light are resuspended when

winds rise. The data in Table 4 confirm this.

38. Plots of means and standard deviations for measured field effluent

suspended solids concentrations and values predicted using the column test

procedure described above are shown plotted in Figure 9. These data graphi-

cally show that the measured mean field concentration of suspended solids is

higher than the predicted concentration from lab column tests for four of the

five comparisons. The predicted values of effluent suspended solids using the

modified McLaughlin analysis described can therefore be considered a minimum

value that can be achieved in the field under the best possible conditions tor

settling (i.e. little turbulence and little or no solids resuspension because

of wind effects). The comparison of predicted values from the column tests

and mean measured field concentrations in Table 4 shows that an adjustment for

turbulence and anticipated solids resuspension caused by wind would be appro-

priate for most cases. Even though the tield data available were limited, the

range of ratios of field values to predicted values shown in Table 4 is a good

25

.A0"

SUSPENDED SOLIDS IN mg/1

0 50 100 150 200 250I I I I

n 37MOBILE 0HARBOR

n =48SAVANNAH 0 AHARBOR

n =18NORFOLK 0HARBOR

n = 29KINGS 0BAY

n = 48BLACK 0 0ROCK 309

LEGENDFIELD MEAN AND STANDARD DEVIATION

0 PREDICTED VALUE FROM COLUMN TEST& PREDICTED VALUE CORRECTED FOR RESUSPENSION

Figure 9. Plot of means and standard deviations for fieldeffluent suspended solids concentrations and predicted

values from column settling tests

indicator of appropriate factors for adjusting the laboratory-measured values

for anticipated turbulence and solids resuspension to produce a predicted

value.

39. A reasonable approach in selecting apprepriate settling efficiency

adjustment factors would be based on both anticipated ponded areas and antici-

pated ponding depths. The level of turbulence is related to advective flow

velocities, which are inversely proportional to ponded surface area and ponded

depth. However, wind effects usually influence flow velocities and turbulence

26

iO

in shallow confined disposal areas to a greater degree than flow rate and

ponded volume. Also, as the ponded area increases, fetch distances for pos-

sible wind-induced waves increase, and the potential for solids resuspension

also increases. As ponded depths and widths increase, the advective velocity

decreases. Also, increasing depth reduces the influence of wave action at the

interface, and the potential for solids resuspension decreases.

40. Field observations of conditions at all the sites indicated light

to moderate wind, with the exception of the Norfolk site. Storm conditions

were experienced at this site during early sampling efforts, and the effluent

suspended solids data that were collected for that sampling period are shown

separately in Table 4. The Norfolk data for storm conditions indicate that

measured field effluent suspended solids can be higher than the values pre-

dicted by the column test by a factor of 10. However, designing for such

extreme conditions would be overly conservative during almost all of the oper-

ating time.

41. The remainder of the cases shown in Table 4 indicate that the

ratios of field-to-laboratory values vary from slightly less than I to 2.27.

A set of recommended settling efficiency adjustment factors was selected based

on these data. [he variables were ponded area (less than or greater than

100 acres) and ponded depth (less than or greater than 2 ft). The recommended

adjustment factors vary trom 1.5 to 2.5 and are presented in Table 5. These

settling efficiency adjustment factors are considered sufficiently conserva-

tive for purposes ot disposal area evaluations under normally encountered wind

conditions.

Table 5

Recommended Settling Efficiency Factors for the Zone Settling

Case for Various Ponded Areas and Depths

Anticipated Average Ponded DepthAnticipated Ponded Area Less than 2 ft 2 ft or Greater

Less than 100 acres 2.0 1.5

Greater than 100 acres 2.5 2.0

27

.0'

42. The values of suspended solids from the lab column tests were cor-

rected for settling efficiency using the appropriate values selected from

Table 5. The adjusted predicted effluent suspended solids concentrations are

shown in Table 4 and are also plotted in Figure 9. In all cases, the adjusted

predicted values are conservative estimates of the effluent suspended solids

concentration, with an average ratio of predicted-to-field values of 1.31.

The adjustment factors for settling efficiency described above are based on

engineering judgment and limited field and laboratory data. For this reason,

it is recommended that the procedures be refined as appropriate as column test

and field data from additional sites become available.

28

PART V: RECOMMENDATIONS

43. Based on the results of testing sediments from Mobile Harbor (com-

posite), Black Rock Harbor, and Yellow Creek, the performance of flocculent

settling tests and the analysis of the data using the McLaughlin approach are

recommended for predicting the suspended solids concentrations in the effluent

from confined disposal areas. For cases in which flocculent settling governs

the entire slurry mass, the procedures recommended by Montgomery (1978) may be

directly applied.

44. For cases in which zone settling describes the settling behavior

of the slurry mass, the modified column test procedures described above should

be used to obtain flocculent settling data for the volume of semi-clarified

supernatant above the interface, and the modified McLaughlin approach should

be used to analyze the data. The column tests should be run in 8-in.-diam

ported columns at slurry concentrations equal to the anticipated field influ-

ent concentrations.

45. Suspended solids concentrations from the quiescent laboratory col-

umn tests should be adjusted upward to account for poorer settling efficiency

resulting from turbulence and solids resuspension under field conditions.

Based on comparisons of column predictions and measured field data from Mobile

Harbor, Savannah Harbor, Norfolk Harbor, Black Rock Harbor, and Kings Bay,

settling efficiency adjustment factors ranging from 1.5 to 2.5 result in con-

servative predictions of effluent suspended solids concentrations and are

recommended. Detailed step-by-step procedures are given in Appendix A.

29

-,A,

REFERENCES

McLaughlin, R. T., Jr. 1959(Dec). "The Settling Properties of Suspensions,"Journal of the Hydraulics Division, Proceedings of the American Society of

Civil Engineers, Vol HY 12.

Montgomery, R. L. 1978. "Methodology for Design of Fine-Grained DredgedMaterial Containment Areas for Solids Retention," Technical Report D-78-56,US Army Engineer Waterways Experiment Station, Vicksburg, Miss.

Palermo, M. R. 1984. "Interim Guidance for Predicting the Quality of Efflu-ent Discharged from Confined Dredged Material Disposal Areas," Technical NotesEEDP-04-1 through 4, Environmental Effects of Dredging Programs TechnicalNotes, (also published as Miscellaneous Paper D-85-1), US Army Engineer Water-ways Experiment Station, Vicksburg, Miss.

. 1986. "Development of a Modified Elutriate Test for Estimating

the Quality of Effluent from Confined Dredged Material Disposal Areas," Tech-nical Report D-86-4, US Army Engineer Waterways Experiment Station, Vicksburg,Miss.

Palermo, M. R., Montgomery, R. L., and Poindexter, M. 1978. "Guidelines forDesigning, Operating and Managing Dredged Material Containment Areas," Techni-cal Report DS-78-10, US Army Engineer Waterways Experiment Station, Vicksburg,Miss.

Palermo, M. R., and Thackston, E. L. 1988. "Field Evaluations of the Qualityof Effluent from Confined Dredged Material Disposal Areas," Technical ReportD-88-1, US Army Engineer Waterways Experiment Station, Vicksburg, Miss.

Shields, F. D., Thackston, E. L., Shroeder, P. R., and Bach, D. P. 1987."Design and Management of Dredged Material Containment Areas to ImproveHydraulic Performance," Technical Report D-87-2, US Army Engineer WaterwaysExperiment Station, Vicksburg, Miss.

Walski, T. M., and Schroeder, P. R. 1978. "Weir Design to Maintain EffluentQuality from Dredged Material Containment Areas," Technical Report D-78-18,US Army Engineer Waterways Experiment Station, Vicksburg, Miss.

30

41

APPENDIX A: RECOMMENDED COLUMN TEST PROCEDURES AND EXAMPLE CALCULATIONS

Recommended Column Test Procedures

1. Settling tests, performed in 8-in.-diam ported columns as shown in

Figure Al, are necessary to provide data for design or evaluation of disposal

areas for retention of suspended solids. These tests are designed to define

the flocculent or zone settling behavior of a particular sediment and to pro-

vide information concerning the volumes occupied by newly placed layers of

dredged material. The test procedures have been refined to obtain data for

use in predicting the concentration of suspended solids in the effluent for

both the flocculent and zone settling case.

2. Sedimentation of freshwater slurries of solids concentrations less

than 100 g/t can generally be characterized by flocculent settling. As solids

concentrations exceed 100 g/1, the sedimentation process may be characterized

by zone settling properties in which a clearly defined interface is formed

between the clarified supernatant water and the more concentrated settled

material. Zone settling also describes the process when the sediment/water

salinity is greater than 3 ppt. The studies described in the main text have

shown that flocculent settling describes the behavior of suspended solids in

the clarified supernatant water above the sediment/water interface for slur-

ries exhibiting an interface.

Apparatus

3. A settling column such as shown in Figure Al should be used. The

test column depth should approximate the effective settling depth of the pro-

posed disposal area. A practical limit on the depth of the test is 6 ft. The

column should be at least 8 in. in diameter with interchangeable sections and

with sample ports at 1-ft or closer intervals in the lower 3 ft and at 1/2-ft

or closer intervals in the upper 3 ft. The column should have provisions to

bubble air from the bottom to keep the slurry mixed during the column filling

period or to recirculate slurry from the bottom of the column to the top.

Flocculent settling test procedure

4. Test data required to design or evaluate a disposal area in which

flocculent settling of the slurry occurs and to predict the concentration of

suspended solids in the effluent can be obtained using the design procedures

Al

4

a

VALVES FOR SAMPLEEXTRACTION

PORTABLE .....MIXER

D..EDGED..MATowIl

ME W :v

OSIIV. ......DISPLAC

..... P .. .. .. .. . ... R.

Figure.. She..cofapaats.o.clun.etlnte.... (after...ntomery.1918

...... .............

described by Montgomery (1978)* and Palermo, Montgomery, and Poindexter

(1978). These procedures allow the prediction of the concentration of the

suspended solids in the effluent, [SS eff , as a function of retention time.

Zone settling test procedure

5. Information required to design or evaluate a disposal area in which

zone settling of the slurry occurs can be obtained by a zone settling test

(Montgomery 1978; Palermo, Montgomery, and Poindexter 1978) performed on sedi-

ment slurry at a concentration equal to the expected mean field influent con-

centration. This test should be continued for a period of at least 15 days to

provide data for estimating disposal area volume requirements. This test is

also used to obtain data for the prediction of effluent suspended solids con-

centrations by defining the flocculent settling behavior of the supernatant

suspended solids. The stepwise procedure for this test is described below.

6. Step 1--slurry preparation and loading. Mix the sediment slurry to

the desired suspended solids concentration in a container with sufficient

voluwe to fill the test column. The test should be performed at the concen-

tration selected to represent the anticipated concentration of the dredged

material influent, Ci . Field studies indicate that for maintenance dredging

in fine-grained material, the disposal area influent concentration will aver-age about 150 g/1. This value may be used for Ci if no better data are

available.

7. Step 2--settling and sampling. Begin extracting samples from the

side ports for determination of suspended solids concentration. For sediments

exhibiting zone settling behavior, an interface will form between the more

concentrated settled material and the semi-clarified supernatant water. The

first sample should be extracted immediately after the interface has fallen

sufficiently below the uppermost port to allow extraction without withdrawing

solids from below the interface. This sample can usually be extracted within

a few hours after initiation of the test, depending on the initial slurry con-

centration and the spacing of ports. Record the time of extraction, port

height, and height of the fluid surface for each port sample taken. As the

interface continues to fall, extract samples from all ports above the inter-

face at regular time intervals. Substantial reductions of suspended solids

will occur during the early part of the test, but reductions will lessen at

* See References at the end of the main text.

A3

longer retention times. Therefore, the sampling intervals can be extended as

the test progresses. A typical sequence of intervals would be 2, 4, 8, 12,

24, 48, 96 hr, etc. The samples should continue to be taken throughout the

15-day test or until the suspended solids concentration of the extracted sam-

ples shows no decrease.

Data analysis

8. For the flocculent settling case, the procedures for data analyses

given by Montgomery (1978) and Palermo, Montgomery, and Poindexter (1978) may

be used. For the zone settling case, flocculent settling describes the set-

tling behavior in the supernatant water above the interface. Therefore, a

flocculent settling data analysis procedure as outlined in the following para-

graphs is required. Example calculations are also shown in the following

paragraphs, using data from a column test conducted on sediment from Savannah

Harbor. This test was conducted at an initial slurry concentration of 99 g/Z

according to the procedures mentioned above.

9. The steps in the data analysis are as follows:

a. Step 1. Arrange the flocculent settling test data from the lab-oratory test as shown in Table Al, and compute values of thedepth of sampling below the fluid surface, z . In computingthe fractions remaining, 0 , the concentration of the firstport sample is considered the initial concentration, SS .

0

b. Step 2. Plot the values of fractions remaining 0(z,t) and zusing the data from the table as shown in Figure A2 forming aconcentration profile diagram. Concentration profiles shouldbe plotted for each time of sample extraction.

c. Step 3. Use the concentration profile diagram to graphicallydetermine percentages removed, R , for the various timeintervals, averaged over any desired ponding depth, D . Thispwis done by graphically determining the areas to the right ofeach concentration profile and its ratio to the total area abovethe depth D . The removal percentagd R iz,

pw

R - Area to right of profile (100) (Al)Total area

d. Step 4. Compute the percentage remaining as simply 100 minusthe percentage removed:

P 100 - R (A2)

A4

.or

Table Al

Selected Observed Flocculent Settling Data, Savannah Harbor Sediment

Sample Depth TotalExtraction Port Head of Sample Suspended Fraction of

Time Height Height Extraction Solids Initial, 0T, hr ft ft z, ft mg/t percent

22 4.7 4.99 0.29 SS - 143 1000

48 4.7 4.95 0.25 77 54

48 4.3 4.95 0.65 90 63

48 4.0 4.95 0.95 90 63

48 3.7 4.95 1.25 78 55

48 3.3 4.95 1.65 106 74

360 4.7 4.73 0.03 30 21

360 4.3 4.73 0.43 30 21

360 4.0 4.73 0.73 33 23

360 3.7 4.73 1.03 30 21

360 3.3 4.73 1.43 29 20

360 3.0 4.73 1.73 32 23

360 2.7 4.73 2.03 31 22

e. Step 5. Compute values for suspended solids for each time ofextraction:

SSt = Pt SS (A3)

Arrange the data as shown in Table A2.

f. Step 6. Plot a relationship for remaining suspended solidsconcentration versus time using the value for each time ofextraction as shown in Figure A3 in Example 1. An exponentialcurve fitted through the data points is recommended.

10. This curve may be used for the prediction of effluent suspended

solids concentrations under good settling conditions for any estimated mean

field retention time. Simply enter the curve with the estimated mean field

A5

ori

PERCENT OF INITIAL CONCENTRATION, 0

0 10 20 30 40 50 60 70 80 90 100

13 T 21.75

0.5 -

1 .0LEGEND

0 21.75 HR13 (3 48 HR

u"Z 96 HR

1.5 168 HRC,=.L 26,4 HR0O 360 HR

T. 0 360--D

T 36..-,s- T =482.5 T = 264

T -96

T = 168 ,3.0

Fiur A2. Concentration profile diagram, Savannah Harbor sediments

Table A2

Percentage of Initial Concentration and Suspended Solids

Concentration Versus Time, Savannah Harbor Sediment

Removal Remaining SuspendedSample Extraction Percentage Percentage Solids

STime, t, hr at t at t mg/k

;22 3 97 139

48 40 61 86

96 61 39 56

168 70 30 43

264 78 22 32

360 81 19 27

A6

I-I

41 l

4

0

C,, N

U,)

0 r.

0 cn.

mwr

LO 41

410

0 4ca -

wL 0)4

IL.

0)

0).

'-

-'-4

A7

retention time, Td , and select the value of suspended solids as predicted by

the column test, SS col Guidance for adjusting the value derived from the

column test for anticipated field settling efficiency and for estimating mean

field retention time is given in the following paragraphs.

Determination of Effluent Suspended Solids Concentration

11. A prediction of the concentration of total suspended solids in the

effluent must consider the anticipated mean retention time in the disposal

area and must account for possible turbulence and resuspension of settled

material because of wind effects. The relationship of supernatant suspended

solids versus time developed from the column settling test is based on quies-

cent settling conditions found in the laboratory. The anticipated mean reten-

tion time in the disposal area under consideration can be used to determine a

predicted suspended solids concentration from the relationship. This pre-

dicted value can be considered a minimum value that can be achieved in the

field assuming little or no turbulence or resuspension of settled material.

However, an adjustment for anticipated poorer settling caused by turbulence

and resuspension is appropriate for typical conditions in dredged material

containment areas. The minimum expected value and the value adjusted for tur-

bulence and resuspension would provide a range of anticipated poorer settling

and suspended solids concentrations for use in predicting the total concen-

trations of contaminants in the effluent. The value adjusted for anticipated

poorer settling and resuspension is

[SS eff] [SScol x SEF (A4)

where

[SSeff] - suspended solids concentration of effluent consideringanticipated resuspension, milligrams suspended solids/

litre of water

[SS col] - suspended solids concentration of effluent as estimated fromcolumn settling tests, milligrams of suspended solids/litre

of water

SEF - settling efficiency adjustment factor selected from Table 5(see main text)

A8

I ! | m Nm

Table 5 summarizes recommended settling efficiency adjustment factors based on

comparisons of suspended solids concentrations as predicted from column set-

tling tests and those obtained from field experiments from a number of sites

with varyirg site conditions. For dredged material exhibiting flocculent set-

tling behavior (freshwater), the concentration of particles in the ponded

supernatant water is on the order of 1 g/I or higher. The turbulence and

resuspension resulting from normal wind conditions will not significantly

increase the concentration. Therefore, an adjustment would not be required

for the flocculent settling case.

Determination of Field Mean Retention Time

12. Estimates of the mean field retention time for expected operational

conditions are required for selecting appropriate settling times in the modi-

fied elutriate test and for determination of suspended solids concentrations

in the effluent. Estimates of the mean retention time must consider both the

theoretical volumetric retention time T - V/Q and the hydraulic efficiency

of the disposal area, defined as the ratio of mean retention time to theoreti-

cal retention time. Mean field retention time, td 0 can be estimated for

given flow rate and ponding conditions by applying a hydraulic efficiency cor-

rection factor to the theoretical detention time, T , as follows:

- T(HECF) (A5)

where

td = mean retention time, hours

T - theoretical retention time, hours

HECF - hydraulic efficiency correction factor (HECF > 1.0) defined asthe inveire of the hydraulic efficiency

The theoretical retention time is

T VP 12.1 AD (12.1) (A6)

A9

i~

where

T - theoretical retention time, hours

V = volume ponded, acre-feet

A p area ponded, acresPD - average depth of ponding, feetp

Q= average influent rate, cubic feet per second12.1 - conversion factor acre-feet/cubic feet per second to hours

13. The hydraulic efficiency correction factor, HECF , can be esti-

mated by several methods. The most accurate estimate is made possible from

field dye tracer data previously obtained at the site under operational condi-

tions similar to those for the operation under consideration. Guidance for

conducting such field tests is presented by Palermo (1984). This approach can

be considered only for existing sites.

14. In the absence of dye tracer data or values obtained from other

theoretical approaches, Shields et al. (1987) showed that the HECF can be

estimated from Equation A7.

= 0.84 1 - exp 0.59 (A7)

Example Calculations

Project information

15. Dredged material from a maintenance project was placed in an exist-

ing disposal site at Savannah Harbor. The site was ponded over an area of

approximately 50 acres. The design calculation using procedures described b-

Montgomery (1978) and Palermo, Montgomery, and Poindexter (1978) indicated

that the surface area was adequate for effective sedimentation if a minimum

ponding depth of 2 ft were maintained. The dredging equipment and anticipated

pumping conditions resulted in a mean flow rate of approximately 30 cfs. A

dye tracer test was run at this disposal site, and the mean field retention

time was 53 hr. Sampling of influent from the dredge pipe indicated that the

influent solids concentration was approximately 107 g/L.

16. The quality of effluent was predicted using the procedures outlined

above.

AA10

Column settling tests

17. Samples from the dredging area were homogenized into a composite

for column settling tests. The test used for prediction of effluent suspended

solids was run at a slurry concentration of 99 g/, equivalent to the influent

slurry concentration. The interface formed early in the test, but the set-

tling velocity was slow, and the initial port sample was taken at 22 hr. Sam-

ples were extracted from all cleared ports at 22, 48, 96, 168, 264, and

360 hr. Partial data for the solids concentrations and calculated values of

z are shown in Table Al. The concentration profile diagram was then con-

structed from the data as shown in Figure A2. Ratios or percentages removed

as a function of time were then determined graphically using the step-by-step

procedure described previously.

18. Since an interface formed in the test, the slurry was undergoing

zone settling. Therefore, the initial supernatant solids concentration,

SS , was assumed to be equal to the concentration of the first port sample0

taken, 143 mg/l. An example calculation for removal percentage for the con-

centration profile at T - 96 hr and D - 2.0 ft using Equation A4 is aspw

follows:

R ' Area right of the profile ((100) = 61%96 Total area Area 01240 (100) ffi (A8)

The areas were determined by planimeter. The percentage remaining at

T - 96 hr is found using Equation A2.

P96 = 100 - R96 = 100 -61 - 39% (A9)

The values for the suspended solids remaining are found using Equation A3.

SS96 96 So 100- (143) - 56 mg/i (AIO)

Values at other times were determined in a similar manner. The data were

arranged in Table A2. An exponential curve fitted to the data for total sus-

pended solids versus retention time is shown in Figure A3.

All

i1

Prediction of effluentsuspended solids concentration

19. A value for effluent suspended solids can be determined for quies-

cent settling conditions using the column test relationship. In this case,

the field mean retention time of 53 hr corresponds to a suspended solids con-

centration of 85 mg/i, as shown in Figure A3. This value should be adjusted

for anticipated field settling efficiency using the factors shown in Table 5.

In this case, for a surface area less than 100 acres and average ponding depth

of 2 ft, the settling efficiency adjustment factor is 1.5. The predicted

total suspended solids concentration in the effluent is calculated using Equa-

tion A4 as

[SSeff] - [SScol x SEF -85 mg/i x 1.5 128 mg/k (All)

Al2

V